Читать книгу Exploring the Solar System - Peter Bond - Страница 21

The direction a spacecraft or other body travels in orbit can be prograde, when a satellite moves in the same direction as the planet (or star) rotates, or retrograde, when it goes in a direction opposite to the planet's (or star's) rotation. All of the planets in the Solar System orbit the Sun in a prograde direction – west to east or counterclockwise as observed from above the Sun's north pole. However, many comets and some satellites move in a retrograde (clockwise) direction.

Various technical terms are used to describe the characteristics of these orbits. The time an object takes to complete one orbit is known as the orbital period. The closest point of an orbit has the prefix “peri” – hence perigee for a satellite of the Earth and perihelion for an object orbiting the Sun. (Helios = Sun.) The furthest point in an orbit has the prefix “ap” – as in apogee and aphelion.

The plane of Earth's orbit around the Sun is called the ecliptic. The orbits of the other planets, comets, and asteroids are tilted to this plane. The angle of the tilt is the orbital inclination. The inclination of a satellite's orbit is measured with respect to the planet's equator. Hence, an orbit directly above the equator has an inclination of 0°, while one passing over a planet's poles has an inclination of 90°.

A planet, asteroid, or comet crosses the ecliptic twice during each orbit of the Sun. The points where an orbit crosses a plane are known as nodes. When an orbiting body crosses the ecliptic plane going north, the node is referred to as the ascending node. Going south, it is the descending node. The line that joins the ascending node and the descending node of an orbit is called the line of nodes.

Figure 1.7 Some important characteristics of a planet's orbit. Here the planet is inferior, i.e. closer to the Sun than Earth. Its orbit is inclined to the ecliptic – the plane of Earth's orbit. The planet's orbit crosses the ecliptic at two nodes – the ascending node (a) and the descending node (d).

(Peter Bond, after Open University)

One of the most important orbital, or Keplerian, elements, is the semi‐major axis, the average distance of an object from its primary (planet or Sun). The shape of the orbit is described by its eccentricity, measured as a number between zero and 1. An eccentricity of zero indicates a circular orbit. A parabola has an eccentricity of 1.

With the invention of the telescope, the possibility arose of finding fainter, more remote planets. The first newcomer, Uranus, was discovered far beyond the orbit of Saturn by William Herschel in 1781. The list was further increased in 1801, when Giuseppe Piazzi found Ceres in the gap between the orbits of Jupiter and Mars. Pallas, Juno, and Vesta – objects in similar orbits to Ceres – were discovered between 1802 and 1807. Since they were clearly much smaller and less substantial than the other planets, they were soon downgraded to “minor planets” or “asteroids” (star‐like objects).

Almost 40 years passed before the eighth planet, Neptune, was discovered by Johann Galle and Heinrich D'Arrest. However, neither Uranus nor Neptune seemed to be following its expected path, suggesting that an even more distant planet might be influencing the movements of its neighbors. The search for this world concluded in 1930 when Clyde Tombaugh observed the tiny image of Pluto on a photographic plate.

For many years, it was generally accepted that there were nine planets, despite growing concerns that Pluto seemed to be too small and lacking in mass to deserve this title. The crunch came in 2003, when Mike Brown discovered 2003 UB313 (now named Eris), an object that is comparable in size to Pluto. With the introduction of ever more sensitive detectors, it seemed likely that there would soon be dozens of Pluto‐sized planets.

Aware that there was no generally accepted definition of the term “planet” and faced with a fierce debate over whether Pluto should be demoted, members of the International Astronomical Union gathered in Prague for the 2006 General Assembly.

After a lengthy discussion, they agreed to define a planet as a celestial body that: (a) is in orbit around the Sun, (b) has sufficient mass for its self‐gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (c) has cleared other objects from the neighborhood of its orbit.

Based on these criteria, the Solar System now consists of eight planets: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune. A new distinct class of objects called “dwarf planets” was also introduced (Figure 1.8). To be classified as a dwarf planet, an object must orbit the Sun and have a nearly round shape. The first dwarf planets to be announced were Ceres (the largest asteroid), Pluto, and Eris, followed by three more. Many others are expected to be discovered in the future.

Figure 1.8 In the “new” Solar System, as defined by the International Astronomical Union in 2006, there are eight planets: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune (shown in order of their distance from the Sun). A new, distinct class of objects called “dwarf planets” includes the largest asteroid, Ceres, and the two largest known Kuiper Belt objects, Pluto and Eris. The relative sizes of the planets and the Sun are shown. Jupiter's diameter is about 11 times that of Earth, and the Sun's diameter is about 10 times that of Jupiter. The distances of the planets are not shown to scale.

(IAU)

This decision has not met with universal approval. One common criticism relates to what exactly is meant by a planet “clearing its neighborhood.” For example, critics argue that Neptune is accepted as a planet, even though many Kuiper Belt objects (including Pluto) cross its orbit. Perhaps, they suggest, it would be more appropriate to use size as a criterion, particularly bearing in mind the diameters of objects that are large enough for gravity to dominate structural strength. There is also some discomfiture with defining Ceres – the largest of the asteroids – as a dwarf planet.

Another complication arises when the current definition is extended to extrasolar planets, i.e. planets orbiting other stars (see Chapter 14). Size is not a useful factor, since many of these planets are similar in size and mass to small, cool “failed stars” known as brown dwarfs.

Instead, astronomers attempt to distinguish between a giant extrasolar planet and a brown dwarf by determining how they were born. A star is formed during the gravitational collapse of a gaseous nebula, whereas a planet is the product of collisions and accretion (snowball‐like growth) between particles in a disk of gas and dust around a central star. Even so, this method of differentiation is difficult to apply, especially in the case of planet‐sized objects that have been flung into interstellar space and no longer orbit any star.